Humans Weren't Always 'So Special,' Expert Says

THURSDAY June 25, 2009 -- A 54-million-year-old skull has yielded the first detailed images of a primitive primate brain.

The 1.5-inch-long skull was from an animal species called Ignacius graybullianus, part of a group of primates known as plesiadapiforms. They evolved in the 10 million years after dinosaurs disappeared from the Earth.

Researchers used CT scans to take more than 1,200 cross-sectional images of the skull and combined them into a three-dimensional model of the brain.

"Most explanations on the evolution of primate brains are based on data from living primates," Mary Silcox, an anthropologist at the University of Winnipeg in Canada and a research associate at the Florida Museum of Natural History at the University of Florida, said in a school news release.

"There have been all these inferences about what the brains of the earliest primates would look like, and it turns out that most of those inferences are wrong," she said.

Ignacius graybullianus was similar to modern primates in terms of diet and tree-dwelling, the researchers report, but its brain was one-half to two-thirds the size of the brain of the smallest modern primates. This suggests that fruit-eating and tree-dwelling, for instance, weren't factors in the evolution of larger brain sizes in primates because "the smaller-brained Ignacius was already doing those things," said Silcox, lead author of a report on the findings, which were published in this week's online issue of the Proceedings of the National Academy of Sciences.

Co-author Jonathan Bloch, a vertebrate paleontologist at the Florida Museum of Natural History, said that "a large and complex brain has long been regarded as one of the major steps that sets primates apart from the rest of mammals."

But at humans' "very humble beginnings, we weren't so special," he said. "That happened over tens of millions of years."